Lower the threshold for Lighthouse accessibility

[choldgraf]

I propose that we lower the threshold of accessibility for lighthouse, and create an issue that states our goal for accessibility on the website. Here is my rationale:

• Currently, our lighthouse CI is failing on every PR. The last time I looked, we were averaging a score of ~.8 while the threshold is .95
• This has created an "expected failures" CI dynamic.
• This means that we aren't actually catching regressions in accessibility, which is what CI is most-useful for doing. It is simply telling us "your accessibility is still below threshold" on every PR.

For this reason:

• I suggest lowering the threshold we use for CI so that it becomes a useful indicator again and can be used to influence PR decisions.
• I suggest we open an issue that defines the goal for lighthouse accessibility we want to hit

[krassowski]

We can set the threshold on per-category basis (as we already do):

jupyter.github.io/.github/workflows/lighthouserc.json

Lines 15 to 18 in 881623c

	"categories:performance": ["error", { "minScore": 0.90 }],
	"categories:accessibility": ["error", { "minScore": 0.95 }],
	"categories:best-practices": ["error", { "minScore": 0.90 }],
	"categories:seo": ["error", { "minScore": 0.95 }]

[krassowski]

I opened #792